AC power adapter having a switchable capacitor

ABSTRACT

A power adapter is provided to supply electric power from an alternating current from an alternating current (AC) power supply to a powered apparatus. It includes a switchable capacitor circuit housed within a housing and including: a first switch path associated with a positive half cycle of the alternating current and a second switch path associated with a negative half cycle of the alternating current, both coupled across the AC power supply. Each switch path includes a switchable capacitor in series with a switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/033,659 filed Jul. 12, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/623,541 filed Jun. 15, 2017, now U.S. Pat. No.10,050,572, which is a continuation of PCT Application No.PCT/US2015/066826 filed Dec. 18, 2015, which is a continuation-in-partof U.S. patent application Ser. No. 14/715,258 filed May 18, 2015, andalso claims the benefit of U.S. Provisional Application No. 62/094,156,filed on Dec. 19, 2014. The entire disclosures of the above applicationsare incorporated herein by reference.

FIELD

The present disclosure relates to a power tool having an electric motor.

BACKGROUND

Some power tools include brushless electric motors. Power tools withbrushless electric motors use a rectifier to convert an alternatingcurrent (AC) input into a direct current (DC) that is used to drive thebrushless electric motor. Power tools with brushless electric motorsalso employ a capacitor to lessen ripple and to provide a current whenthe AC input voltage is unable to do so.

During certain operating conditions, such as a high load, high voltagetransients may be generated in a DC power bus that transmits directcurrent to the electric motor. The high voltage spikes can damageelectronic components within the power tool. Accordingly, there is aneed for a power tool that mitigates high voltage spikes withouteffecting normal operation of the power tool.

This section provides background information related to the presentdisclosure, which is not necessarily prior art.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

In an aspect of the present disclosure, the present disclosure may bedirected toward a power tool configured to receive power from a powersupply. The power tool may comprise: an electric motor, a rectifier, aswitching arrangement, a switch path, and a switch control circuit. Therectifier may be configured to receive an alternating current from analternating current (AC) power supply and output a rectified signalsupplied to a DC power bus. The switching arrangement may have aplurality of motor switches connected electrically between the DC powerbus and the electric motor, and may operate to deliver electric powerfrom the DC power bus to the electric motor. The switch path may beelectrically coupled in parallel with the rectifier on the DC power bus.The switch path may include an auxiliary capacitor in series with aswitch and a state of the switch may control a discharging path for theauxiliary capacitor. The switch control circuit may be configured todetect voltage associated with at least one of the AC power supply orthe DC power bus and to control state of the switch in accordance withmagnitude of the detected voltage.

In an aspect of the present disclosure, the switch control circuit mayinclude a sensing circuit and a comparator. The sensing circuit may beconfigured to detect the voltage across the DC power bus, and thecomparator may be configured to compare the voltage detected with apredetermined threshold and output a signal to the switch to control thestate of the switch.

In another aspect of the present disclosure, the sensing circuit mayinclude a voltage divider that is electrically coupled to the DC powerbus and the comparator. The comparator may include a transistor and adiode. The diode may be coupled between the DC power bus and the emitterof the transistor, the base of the transistor may be coupled to thevoltage divider, and the collector of the transistor may be coupled tothe switch to control the state of the switch.

In yet another aspect of the present disclosure, the power tool mayfurther comprise a DC bus capacitor. The DC bus capacitor may beelectrically coupled in parallel with the rectifier and electricallyconnected between the rectifier and the switching arrangement. Thecapacitance of the DC bus capacitor may be less than capacitance of theauxiliary capacitor.

In an aspect of the present disclosure, the DC bus capacitor may have acapacitance of about 5 μF to 20 μF.

In another aspect of the present disclosure, the switch control circuitmay open the switch to prevent current flow through the discharge pathwhen the detected voltage is less than or equal to a first voltagethreshold and close the switch to allow current flow through thedischarge path when the detected voltage is greater than a secondvoltage threshold greater than the first voltage threshold.

In yet another aspect of the present disclosure, the electric motor maybe a brushless motor.

In an aspect of the present disclosure, the switch control circuit maybe coupled to at least one node of the DC power bus.

In another aspect of the present disclosure, the switch control circuitmay include a sensing circuit and a comparator. The sensing circuit maydetect voltage across the auxiliary capacitor, and the comparator may beconfigured to compare the voltage detected with a predeterminedthreshold and output a signal to the switch to control the state of theswitch.

In yet another aspect of the present disclosure, the power tool mayfurther comprise a first housing and a second housing. The first housingmay house the electric motor and the switching arrangement, and thesecond housing separate from the first housing may house the switch pathand the switch control circuit. The second housing may be configured toconnect to the first housing to electrically couple the switch path inparallel with the DC power bus.

In an aspect of the present disclosure, the present disclosure may bedirected toward a power tool configured to receive power from a powersupply. The power tool may comprise: an electric motor, a power bus, aswitching arrangement, a DC bus capacitor, an auxiliary switch path, anda switch control circuit. The power bus may include positive andnegative lines. The switching arrangement may have a plurality of motorswitches and may be operable to deliver electric power from the powerbus to the electric motor. The DC bus capacitor may be electricallycoupled across the power bus. The auxiliary switch path may beelectrically coupled in parallel with the DC bus capacitor. Theauxiliary switch path includes an auxiliary capacitor in series with aswitch. The capacitance of the DC bus capacitor may be less thancapacitance of the auxiliary capacitor. The switch forms a first currentpassageway through the auxiliary capacitor to charge the auxiliarycapacitor when in a first state and forms a second current passagewaythrough the auxiliary capacitor to discharge the auxiliary capacitorwhen in a second state. The switch control circuit may be configured todetect voltage of the power bus and to control state of the switch inaccordance with magnitude of the detected voltage.

In an aspect of the present disclosure, the switch may include atransistor and a switch control circuit that may be configured toactivate and deactivate the transistor based on the detected voltage.

In another aspect of the present disclosure, in the first state, thetransistor may electrically decouple the auxiliary capacitor from thepower bus to have current flow through the auxiliary capacitor via thefirst current passageway, and in the second state, the transistor mayelectrically couple the auxiliary capacitor to the power bus to havecurrent flow through the auxiliary capacitor via the first currentpassageway and the second current passageway.

In yet another aspect of the present disclosure, the switch controlcircuit may include a sensing circuit and a comparator. The sensingcircuit may be configured to detect the voltage across at least one ofthe power bus and the auxiliary capacitor, and the comparator maycompare the voltage detected with a predetermined threshold and output asignal to the switch to control the state of the switch.

In an aspect of the present disclosure, the switch control circuit maycontrol the switch in the first state when the detected voltage is lessthan or equal to a first voltage threshold and control the switch in thesecond state when the detected voltage is greater than a second voltagethreshold greater than the first voltage threshold.

In another aspect of the present disclosure, the DC bus capacitor mayhave a capacitance of about 10 μF to 20 μF.

In yet another aspect of the present disclosure, the power tool mayfurther comprise a housing, a power supply interface, and an AC poweradapter. The housing may house the electric motor, the switchingarrangement, and the DC bus capacitor. The power supply interface may bepositioned along a surface of the housing. The AC power adapter may beconfigured to attach to the power supply interface and may include anauxiliary capacitor circuit. The auxiliary capacitor circuit may includethe auxiliary switch path and the switch control circuit. The auxiliaryswitch path may be electrically coupled to the power bus when the ACpower adapter is attached to the power supply interface and may beelectrically decoupled from the power bus when the AC power adapter isdetached from the power supply interface.

In an aspect of the present disclosure, the present disclosure may bedirected toward a power tool configured to receive power from a powersupply. The power tool may comprise: a brushless electric motor; arectifier, a switching arrangement, an auxiliary switch path, a DC buscapacitor, and a switch control circuit. The rectifier may be configuredto receive an alternating current from an alternating current (AC) powersupply and may be operable to convert the alternating current to arectified current supplied to a DC bus. The switching arrangement mayhave a plurality of motor switches connected electrically between therectifier and the brushless the electric motor, and may operate todeliver the rectified current from the rectifier to the electric motor.The auxiliary switch path may be electrically coupled across the DC busin parallel with the rectifier. The auxiliary switch path may include anauxiliary capacitor in series with a transistor. The DC bus capacitormay be electrically coupled in parallel with the rectifier. Thecapacitance of the DC bus capacitor may be less than capacitance of theauxiliary capacitor. The switch control circuit may be configured todetect voltage associated with the power supply or the DC bus and tocontrol state of auxiliary capacitor by way the transistor in accordancewith magnitude of the detected voltage. The switch control circuit maycontrol the transistor in an OFF state to charge the auxiliary capacitorwhen the detected voltage is less than or equal to a first voltagethreshold and control the transistor in an ON state to discharge theauxiliary capacitor when the detected voltage is greater than a secondvoltage threshold greater than the first voltage threshold.

In an aspect of the present disclosure, the transistor is an IGBT withan anti-parallel diode.

In another aspect of the present disclosure, the auxiliary capacitordischarges to the DC bus when the transistor is in the ON state.

In an aspect of the present disclosure, the present disclosure may bedirected toward a power adapter for use with a power tool and configuredto supply electric power from an alternating current power supply to thepower tool. The power tool may include an electric motor. The poweradapter may comprise: a housing and an auxiliary capacitor circuit. Theauxiliary capacitor circuit may be housed within the housing and mayinclude an auxiliary capacitor, a switch in series with the auxiliarycapacitor, and a switch control circuit. A state of the switch maycreate either a charging path for the auxiliary capacitor or both thecharging path and a discharging path for the auxiliary capacitor. Theswitch control circuit may be configured to detect voltage associatedwith the power supply or the power tool and to control state of theswitch in accordance with magnitude of the detected voltage.

In an aspect of the present disclosure, the switch control circuit mayinclude a sensing circuit and a comparator. The sensing circuit may beconfigured to detect the voltage across at least one of a DC busprovided in the power tool and the auxiliary capacitor. The comparatormay compare the voltage detected with a predetermined threshold andoutput a signal to the switch to control the state of the switch.

In another aspect of the present disclosure, the sensing circuit mayinclude a voltage divider that is electrically coupled to the comparatorand may be configured to electrically couple to the DC bus. Thecomparator may include a transistor and a diode. The diode may beconfigured to electrical couple between the DC bus and the emitter ofthe transistor, the base of the transistor may be coupled to the voltagedivider, and the collector of the transistor may be coupled to theswitch to control the state of the switch.

In yet another aspect of the present disclosure, the power adapter mayfurther comprise one or more terminals that may be electrically coupledto the auxiliary capacitor circuit and configured to connect to aninterface provided on the power tool. With the terminals connected tothe interface of the power tool, the auxiliary capacitor and the switchform an auxiliary switch path that may be electrically coupled across aDC bus of the power tool, and the switch control circuit may beelectrically coupled to the DC bus for detecting the voltage along theDC bus.

In an aspect of the present disclosure, the power adapter may furthercomprise a rectifier and a DC bus capacitor. The rectifier may beconfigured to receive an alternating current from the alternatingcurrent power supply and may be operable to convert the alternatingcurrent to a rectified current supplied to a DC bus. The DC buscapacitor may be electrically coupled in parallel with the rectifier,and the capacitance of the DC bus capacitor may be less than capacitanceof the auxiliary capacitor.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only, and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a perspective view of a power tool, such as a grinder;

FIG. 2 is an example motor control system which may be employed by thepower tool;

FIGS. 3A, 3B, and 3C are graphs illustrating voltage and currentwaveforms for different DC bus capacitors;

FIG. 4 is an example motor control system of the present disclosure thathas an auxiliary switch path;

FIG. 5 is a graph of voltage and current waveforms of a power supplyproviding power to the power tool;

FIG. 6 is a graph comparing voltage waveforms of the motor controlsystems of FIG. 4 (solid line) and FIG. 2 (dotted line);

FIG. 7 illustrates an example of the auxiliary switch path and a switchcontrol circuit;

FIG. 8 illustrate an example circuit of the auxiliary switch path andthe switch control circuit;

FIG. 9 is a perspective view of a power tool having a power supplyinterface;

FIG. 10 is a perspective view of an AC power adapter;

FIG. 11 is a block diagram of a motor control system of the power tooland the AC power adapter, where the AC power adapter includes anauxiliary capacitor circuit having an auxiliary switch path and answitch control circuit in a second embodiment of the present disclosure;

FIG. 12 depicts an exemplary power system including a power tool and anadaptor disposed between the power tool and an AC power supply;

FIG. 13 depicts a circuit block diagram of the power system, accordingto an embodiment;

FIG. 14 depicts an exemplary circuit diagram of an auxiliary capacitorcircuit disposed within the adaptor, according to an embodiment;

FIG. 15 depicts the circuit diagram of FIG. 14 including charge anddischarge current paths for the auxiliary capacitor, according to anembodiment;

FIG. 16 depicts an exemplary circuit diagram of an alternative auxiliarycapacitor circuit disposed within the adaptor, according to anembodiment;

FIG. 17 depicts the circuit diagram of FIG. 16 including charge anddischarge current paths for the auxiliary capacitor within a positive AChalf cycle, according to an embodiment;

FIG. 18 depicts the circuit diagram of FIG. 16 including charge anddischarge current paths for the auxiliary capacitor within a negative AChalf cycle, according to an embodiment; and

FIG. 19 depicts an exemplary block circuit diagram of half-cycledetection and switch control unit, according to an embodiment.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 1 depicts an example power tool 10. In this example embodiment, thepower tool 10 comprises a housing 12 having an elongated shape. A usercan grasp the power tool 10 by placing the palm of the user's hand overand around the housing 12. An output member 18 is positioned at one end12-1 of the housing 12 and comprises a right angle gearset 20 thatdrives a rotating disk 22. In this example embodiment, the rotating disk22 comprises a grinder disk.

The rotating disk 22 may be removed and replaced with a new rotatingdisk. For example, a user of the power tool 10 may replace the existingrotating disk 22 with a new rotating disk after the existing rotatingdisk 22 wears out. An adjustable guard 24 may cover at least a portionof the rotating disk 22 to obstruct sparks and debris generated duringoperation of the power tool 10.

The housing 12 has a first portion 14 and a second portion 16. The firstportion 14 and the second portion 16 may be secured together with screws26, and enclose an electric motor 28 and electronic circuit componentsthat drive the output member 18. While the present description isprovided with reference to a brushless electric motor, the electricmotor 28 may be any type of electrical motor capable of driving theoutput member 18. A power cord 30 is connectable to an AC power supplyand is positioned at an opposite end 12-2 of the housing 12. The powercord 30 provides power to the electric motor 28 and the electroniccircuit components of the power tool 10.

The first portion 14 further includes a power on/off switch 32 and aspindle lock switch 34. Operating the power on/off switch 32 in ON andOFF positions turns the electric motor 28 ON or OFF, respectively.Pressing and holding the spindle lock switch 34 enables the user tochange the rotating disk 22. A plurality of narrow slot openings 36 ofthe first 14 and second 16 portions allow for venting of the electricmotor 28 and the electronic circuit components. The one end 12-1 of thehousing 12 also includes a threaded opening 38 for selectively attachinga side-handle (not shown) to enable two-handed operation.

While the present description is provided with reference to a grinder,it is readily understood that the broader aspects of the presentdisclosure are applicable to other types of power tools, including butnot limited to sander, drill, impact driver, tapper, fastener driver,and saw. For example, the power tool 10 may include a chuck that isconfigured to receive a drill bit or a screw driver bit, therebyallowing the power tool 10 to be used as a power drill or a power screwdriver. In another example embodiment, the output member 18 may beremoved and replaced with another output member that may be moresuitable for a drill, a screw driver, or any other power tool.

FIG. 2 depicts a schematic that illustrates an example of a motorcontrol system 100 that may be employed by the power tool 10. The motorcontrol system 100 is comprised generally of a controller 102, aswitching arrangement 104, a driver circuit 106, a rectifier 108, and aDC bus capacitor 110, and a power supply regulator 112. The motorcontrol system 100 may further include position sensors 114, such asHall Effect sensors that are configured to detect rotational motion ofthe electric motor 28 and generate a signal indicative of the rotationalmotion. The signal may have a periodic waveform whose magnitude may varyin accordance with the rotational position of the electric motor 28. Itshould be noted, that other types of positional sensors may bealternatively utilized and should not be limited to Hall effect sensors.

An AC power supply 116 delivers an alternating current to the rectifier108 through, for example, the power cord 30. The rectifier 108 convertsthe alternating current into a direct current that is outputted to a DCbus 115 (i.e., power line/bus). The output of the rectifier 108 may be apulsating DC signal and not a pure DC signal.

The DC bus capacitor 110 is electrically connected in parallel with therectifier 108 on the DC bus line 115. The switching arrangement 104 iscoupled to DC bus 115 line and receives rectified voltage from therectifier 108 and the DC bus capacitor 110. The switching arrangement104 includes a plurality of motor switches that, when switched on,deliver the DC current to the electric motor 28. The motor switches maybe IGBTs or FETs. The switching arrangement 104 may be further definedas a three-phase inverter bridge although other arrangements arecontemplated by this disclosure.

The driver circuit 106 interfaces with the motor switches of switchingarrangement 104. The driver circuit 106 controls the state of the motorswitches. In the example embodiment, the driver circuit 106 is shown asbeing separate from the switching arrangement 104. Alternatively, thedriver circuit 106 and the switching arrangement 104 may be a singleintegrated circuit which may be commercially available from variousmanufactures. For example, the switching arrangement 104, which mayinclude IGBTs, and the driver circuit 106 may be a part of an integratedpower module.

The controller 102 interfaces with the driver circuit 106 and maygenerate PWM signals to control the electric motor 28. In thisembodiment, the controller 102 receives power from the power supplyregulator 112. In an alternate embodiment, the controller 102 mayreceive power directly from the rectifier 108.

The power supply regulator 112 is electrically connected in parallelwith the rectifier 108 and operates to power the driver circuit 106 viathe power on/off switch 32. The power on/off switch 32 is positionedbetween the power supply regulator 112 and the driver circuit 106.

When the power on/off switch 32 is switched to the ON-position, thedriver circuit 106 receives power from the power supply regulator 112.When the driver circuit 106 receives power, the driver circuit 106 isable to control the state of the motor switches and the electric motor28 is on.

Conversely, when the power on/off switch 32 is switched to theOFF-position, the driver circuit 106 does not receive power from thepower supply regulator 112. When the driver circuit 106 does not receivepower, the driver circuit 106 is not able to control the state of themotor switches and the electric motor 28 is off.

In the illustrated example, the power on/off switch 32 is electricallyconnected between the power supply regulator 112 and the driver circuit106. Thus, the power on/off switch 32 is positioned such that the powerfrom the AC power supply 116 does not pass through the power on/offswitch 32. Furthermore, the current being drawn by the electric motor 28does not pass through the power on/off switch 32. The current passingthrough the power on/off switch 32 is the current being drawn by thedriver circuit 106 and the current being drawn by the driver circuit 106is lower than the current being drawn by the electric motor 28. It mustbe understood, however, that in an alternative embodiment, the on/offswitch 32 may be a current-carrying switch disposed, for example, on theDC bus line 115 between the rectifier 108 and the switching arrangement104.

In an embodiment, the DC bus capacitor 110 may be a link capacitorhaving a relatively small capacitance and does not significantlysmoothen the full-wave rectified AC voltage. The DC bus capacitor 110may be a bypass capacitor that removes the high frequency noise from thebus voltage.

FIGS. 3A-3B highlights the advantages of using a small DC bus capacitor110 in the power tool 10. FIG. 3A, in an embodiment, depicts the voltagewaveform using a relatively large DC bus capacitor 110 (e.g.,approximately 400 to 1000 μF) and the associated current waveform. FIG.3B depicts the voltage waveform using a relatively mid-sized DC buscapacitor 110 (e.g., approximately 50 to 200 μF) and the associatedcurrent waveform. FIG. 3C depicts the voltage waveform using a verysmall DC bus capacitor 110 (e.g., approximately 10 to 30 μF) and theassociated current waveform. It is noted that these DC bus capacitorvalues depend on many factors, most notably the power tool power outputrequirement. It is noted that exemplary capacitor values provided hereinare implemented and tested in conjunction with a circuit as shown inFIG. 2 with max power out of 1.5 to 2 kW.

As shown in FIG. 3A, when using a large DC bus capacitor 110, thecurrent is drawn from the DC bus capacitor 110 for a large portion ofeach cycle during the capacitor discharge. Thus, current drawn from theAC power supply during each cycle occurs within a small window, whichcreates a significant current spike. To obtain a constant RMS currentof, for example, 10A from the AC power supply, the current level withinthe small window increases substantially, which creates the currentspikes.

The current spikes in this arrangement are undesirable for two reasons.First, the power factor of the tool becomes low, and the harmoniccontent of the AC current becomes high. Secondly, for a given amount ofenergy transferred from the AC source to the tool, the RMS value of thecurrent will be high. The practical result of this arrangement is thatan unnecessarily large AC circuit breaker is required to handle thecurrent spikes for a given amount of work.

By comparison, as shown in FIG. 3B, when using a mid-sized, the currentis drawn from AC power supply within each cycle occurs within a broadertime window, which provides a lower harmonic content and higher powerfactor. Similarly, as shown in FIG. 3C, when using an even smallercapacitor, the current drawn from the capacitor is very small (almostnegligible) within each cycle. Thus, the current drawn from the AC powersupply is even broader within each cycle. This provides an even lowerharmonic context and a much higher power factor in comparison to FIG.3A.

Additionally, although small DC bus capacitors provide a lower averagevoltage to the motor control system, it is indeed possible to obtain ahigher power output from the AC power supply. In particular, the smallercapacitors enable more power to be drawn from the AC power supply with alower harmonic context and higher power factor.

For more details on the benefits and advantages of using a small DC buscapacitor in a AC powered or hybrid AC/DC powered power tool system,reference is made to U.S. patent application Ser. No. 14/715,258 filedMay 18, 2015, which is incorporated herein by reference in its entirety.

While using a relatively small DC bus capacitor 110 in the system ofFIG. 2 provides many advantages, it may sometimes be desirable to use alarger value capacitor in certain applications. For example, it may bedesirable to use a larger DC bus capacitor value in certain applicationsor conditions, e.g., based on changes in load, changes in speed, changesin power supply voltage, etc. In a specific example, it was found by theinventors of this application that certain power sources, such ascertain gas or diesel AC generators, produce transients in the voltagewaveform, particularly while operating at high load, when coupled to anAC corded power tool having a brushless motor. The transients in thevoltage may cause system and component failure. It was further found bythe inventors, as described in detail below, that using a larger DC buscapacitor reduces the voltage transients on the DC bus line, thusreducing risk of damage to the power tool components.

Thus, according to an embodiment of the present disclosure, a switchableauxiliary smoothing capacitor is provided in parallel to the DC buscapacitor. The auxiliary capacitor may be selectively activated via aswitch provided in series with the auxiliary capacitor. In anembodiment, the auxiliary capacitor may be selectively activated via theswitch when the voltage transients from the power source (such as apower generator) cause the voltage on the DC bus line to exceed apredetermined threshold.

FIG. 4 depicts a motor control system 140 having an auxiliary switchpath 150. The auxiliary switch path 150 is electrically coupled inparallel with the rectifier 108 and the DC bus capacitor 110, and iselectrically connected between the rectifier 108 and the switchingarrangement 104. The auxiliary switch path 150 includes an auxiliarycapacitor 152 and a switch 154 positioned in series with the auxiliarycapacitor 152.

The switch 154 may be a controlled switch, such as a MOSFET or an IGBT,in an embodiment. The switch 154 may be controlled by softwareprogrammed into the controller 102 or another programmablemicrocontroller. Alternatively, the switch 154 may be controlled byhardware, such a switch control circuit 156, described in detail below.When the switch 154 is closed, the auxiliary capacitor 152 is connectedin parallel to the DC bus capacitor 110. An exemplary application andimplementation of the switchable auxiliary smoothing capacitor isdescribed herein.

FIG. 5 depicts a graph of the voltage waveform obtained from a powergenerator during use by an AC corded power tool having a brushlessmotor. As shown herein, at no load the AC voltage exhibits harmonicshaving a normal nominal peak voltage. However, during use with someload, current transients cause large spikes in voltage that are almosttwice the normal nominal peak voltage at no-load. These voltage spikesdamage the electronic components of the power tool and producesignificant audible noise.

As depicted in the waveform diagram of FIG. 6 using of a relativelylarge capacitor (i.e., in the 100 to 1000 hF range) reduces the voltagetransients on the bus and produces better voltage harmonics. The reasonfor this is that in brushless DC motors, such as motor 28, pulse-widthmodulation (PWM) control of the switching arrangement 104 regulates theamount of current provided to the motor 28. The PWM control of themotor, however, has an adverse effect on the bus voltage if the powersupply is reactive. In particular, transients in the bus voltage V is afunction of change in current over time and inductance of the powersupply,

$V = {L{\frac{\Delta\; I}{\Delta\; t}.}}$In non-reactive power generators, such as AC mains, the inductance isvery small compared to AC generators, and therefore the bus voltagefollows the nominal voltage harmonics of the power supply regardless ofthe current. By contrast, in sources such as power generators andalternators, which are reactive electrical systems, there is inherentinductance L that affects the voltage bus transients in response tosudden changes in load current.

For the motor control system 140 of the present disclosure, theauxiliary capacitor 152 provides a path for unused load current,eventually damping down the rise in bus voltage. The rise in voltageacross the capacitor is defined by,

${\frac{\Delta\; V}{\Delta\; t} = \frac{I}{C}},$where I is the amount of current being absorbed by the capacitor. Ascurrent decays over the time, by selecting an appropriate value forcapacitor, maximum bus voltage can be reduced down to a desired level.In an embodiment, the auxiliary capacitor 152 reduces the voltagetransients caused by load current fluctuations by 50%.

In an exemplary embodiment, the capacitance of the auxiliary capacitor152 is greater than that of the DC bus capacitor 110. In particular, theauxiliary capacitor 152 may have a capacitance that is 5 to 40 timeslarger than that of the DC bus capacitor 110 depending on the powersupply voltage. For example, if the DC bus capacitor 110 is in the rangeof 10 to 20 μF, the auxiliary capacitor 152 may be, for example, 50 to200 μF for power tools having rated voltage of approximately 120V and400-800 μF for power tools having rated voltage of approximately 230V.It should be readily understood that the capacitance of the auxiliarycapacitor 152 and the DC bus capacitor 110 are not limited to the valuesdescribed herein and may be set to other suitable values.

FIG. 7 depicts an example embodiment of the auxiliary switch path 150and the switch control circuit 156. The switch control circuit 156selectively activates or deactivates the auxiliary capacitor 152. Theswitch 154 is provided as a transistor 200, such as an insulated-gatebipolar transistor (IGBT) that has an anti-parallel diode 202. The diode202 is configured to form a charge path (i.e., a first path) indicatedby arrow 204 and the IGBT 200 is configured to form a discharge path(i.e., second path) indicated by arrow 206. The switch 154 controls thefollow of current through the auxiliary capacitor 152 via the chargepath and/or the discharge path. It is readily understood that otherswitching mechanisms may be utilized and that the switch 154 should notbe limited to the components described herein.

The switch control circuit 156 includes a sensing circuit 208 and acomparator 210. The sensing circuit 208 senses voltage along the DC bus115. For example, in the example embodiment, the sensing circuit 208detects the voltage across the capacitor 152. Alternatively, the sensingcircuit 208 may alternatively be arranged to sense the voltagedifference between the positive and negative nodes of the DC bus 115(denoted as +DC and −DC herein).

The sensing circuit 208 provides the sensed voltage to the comparator210, which compares the voltage to a predetermined voltage thresholdV_(clamp). If the sensed voltage exceeds the voltage thresholdV_(clamp), the comparator 210 output turns ON the transistor 200, whichcouples the auxiliary capacitor 152 to the DC bus 115 and discharges theauxiliary capacitor 152. More particularly, the IGBT 200 allows currentto flow through the discharge path 206 such that the auxiliary capacitor152 discharges built up charge to reduce the bus voltage transients. Inaddition to current flowing through the discharge path, current may alsoflow through the charge path.

When the sensed voltage is lower than the voltage threshold, the IGBT200 is turned OFF and current flows through the diode 202 via the chargepath 204. Specifically, in this embodiment, the auxiliary capacitor 152is charged by current through the anti-parallel diode 202 during everypeak of every half cycle of the bus voltage when the bus voltage exceedsthe voltage of the auxiliary capacitor 152. This keeps the auxiliarycapacitor 152 in fully charged state. Current to the auxiliary capacitor152 during normal operation is very nominal and may be determined by thesum of capacitor's internal discharge and the current through thesensing circuit 208. The current is small enough that it does notinfluence Power-Factor or harmonics of the system.

The voltage threshold for opening and closing the switch 154 (e.g.,turning transistor 200 OFF and ON) may be provided as two separatethresholds. For example, a first voltage threshold may be designated forhaving the switch 154 open when the detected voltage is less than thefirst voltage threshold and a second voltage threshold may be designatedfor closing the switch 154 when the detected voltage is greater than orequal to the second voltage threshold. The second voltage threshold isgreater than the first voltage threshold.

FIG. 8 depicts an exemplary circuit diagram for the switch controlcircuit 156. In this embodiment, the comparator 210 is formed around pnptransistor Q306. A zener diode D306 forms a reference voltage along withresistor R332. The reference voltage is applied to the emitter oftransistor Q306. The emitter voltage is maintained at zener voltage(−5.1V) with respect to bus voltage. A filter capacitor C315 is providedfor the reference voltage.

In an embodiment, the sensing circuit 208 includes resistors R334, R333and R323 which form a voltage divider. Using this voltage divider,actual capacitor voltage is applied to the base of transistor Q306. Inthis arrangement, transistor Q306 remains reverse-biased until thresholdvoltage (V_(clamp)) is met across the auxiliary capacitor 152. Nocollector current flows during this time and resistor R326 keeps thegate to emitter voltage of IGBT 200 at zero. The auxiliary capacitor 152is constantly charged to the peak AC voltage using the diode 202. Assoon as the capacitor 152 charges beyond the threshold voltage V_(clamp)(e.g., second voltage threshold), transistor Q306 forward biases. Thethreshold voltage V_(clamp) may be, for example, 200 Vdc for a 120Vsystem and 375 Vdc for a 230V system. This causes transistor Q307 toturn on and bypass resistor R334, which in turn lowers the thresholdvoltage to approximately 180 Vdc (e.g., first voltage threshold),allowing approximately 20 Vdc hysteresis. At this time, diode D306 alsoforward biases to charge the gate of the IGBT 200. Gate of IGBT 200(including miller capacitance) is estimated to charge to 18V within 700μs. Diode D307 acts to ensure gate voltage never exceeds 18V. Once theauxiliary capacitor 152 voltage is lowered below 180V, transistor Q306reverse biases causing transistor Q308 to turn ON. Transistor Q308,along with resistors R330 and R331, forms a pnp base biased circuit,which acts to discharge the IGBT 200 gate quickly. Resistors R327, R238,and R337 control hysteresis of the switch 154.

In an embodiment, when input voltage is very low, IGBT 200 remainsturned OFF, hence the capacitor-diode combination acts like a peakdetector with a nominal discharge through its sense resistors. As soonas the voltage transients start to occur and capacitor voltage riseshigh enough to exceed comparator's high threshold voltage, IGBT 200turns ON and connects capacitor 152 to the DC bus 115 for discharge.While IGBT 200 is on, capacitor 152 and the bus voltages follow eachother very closely. After the bus voltage collapses low enough to hitthe lower threshold voltage of the comparator 210 (i.e., transistorQ306), IGBT 200 turns OFF and disconnects the auxiliary capacitor 152once again.

It is noted that while the switch control circuit 156 disclosed hereinis implemented using a voltage sensor and a comparator, the same switchcontroller may be implemented in software, e.g., a micro-controllercoupled to the bus line that read the voltage and compares the voltageto a predetermined voltage level.

It is further noted that while in the exemplary embodiments herein asingle auxiliary capacitor 152 is utilized, multiple auxiliarycapacitors 152 may be disposed in parallel. In addition, the auxiliarycapacitors 152 may be switched on or off individually or in tandem. Thisarrangement provides system flexibility in the amount of totalcapacitance that is to be provided to the DC bus 115.

In the example embodiment, the auxiliary switch path is positionedbetween the rectifier 108 and the switching arrangement 104 within thehousing 12 of the power tool 10. Alternatively, the auxiliary switchpath and the switch control circuit may be positioned outside of thehousing of the power tool and within an adaptor that is configured toconnect to the power tool. In one such embodiment, the auxiliary switchpath coupled across the DC bus line, as described herein with referenceto FIGS. 9-11.

More particularly, with reference to FIG. 9, a power tool 300 (e.g.,miter saw) includes a power supply interface 302 that is configured toconnect to a power supply such as DC batteries or AC power. Power tool300 is an example of an AC/DC power tool configured to receive powerfrom an AC power source, or one or more DC battery packs, as describedin detail in WO 2015/179318, which is incorporated herein by referencein its entirety. As an example, the power supply interface 302 may beconfigured to include, one or more DC battery interfaces 304. A pair ofDC battery packs (not shown) are configured to connect to the batteryinterface 304. DC battery packs may be, for example, 60V Max DC batterypacks, or convertible battery packs configured to output 60V DC Maxbattery power. Accordingly, the power tool 300 may be powered by morethan one DC battery pack power supply that, when connected in series,together have a high rated voltage that corresponds to the AC ratedvoltage of the main power supply.

The power supply interface 302 may also include an AC supply interface306 for receiving AC power. With additional reference to FIG. 10, thepower supply interface 302 may be configured to engage with an AC poweradapter 308. The AC power adapter 308 includes a housing 310 and a cord312 including a two or three pronged plug (not shown) at a first end andis coupled to the housing 310 at a second end. The housing 310 includesa pair of DC power tool interfaces 314 that are substantially equivalentin shape and size as the DC battery interface 304 of the power supplyinterface 302. The housing 310 also includes a receptacle 316 positionedbetween the pair of DC power tool interfaces 314. In the exampleembodiment, the receptacle 316 has three prongs that are configured toconnect to the AC supply interface 306. Alternatively, the receptaclemay be two prong or have other suitable configuration.

The AC power adapter 308 also includes a circuit disposed in the housing310. FIG. 11 is an example embodiment of the AC power adapter 308coupled to the power supply interface 302. The power tool 300 includes amotor control system 320 that is substantially similar to the motorcontrol system 100 of FIG. 2. The power supply interface 302 isconfigured to electrically couple the motor control system 320 to an ACpower supply by way of the AC power adapter 308.

The AC power adapter 308 includes an auxiliary capacitor circuit 330that includes the auxiliary switch path 150 and the switch controlcircuit 156 (not shown in FIG. 11). For example, the auxiliary capacitorcircuit 330 may have the same circuit configuration as the auxiliaryswitch path 150 and the switch control circuit 156 illustrated in FIG. 7or 8.

The AC power adapter 308 and the power supply interface 302 both includeterminals B+ (T_(B+)), B− (T_(B−)), and AC (T_(AC)). Terminals B+ and B−electrically connect to the DC bus 115 and terminal AC electricallyconnects the AC power supply to the rectifier 108. By way of internalwiring that runs thru the AC power adapter 308, the power supplyinterface 302, and the power tool 300, the auxiliary capacitor circuit330 is electrically coupled to the DC bus 115. Specifically, theauxiliary capacitor circuit 330 is electrically coupled in parallel withthe rectifier 108 and electrically connected between the rectifier 108and the switching arrangement 104.

In the second embodiment, the rectifier 108 and the DC bus capacitor 110are positioned within the power tool 300 as part of the motor controlsystem 320. Alternatively, the rectifier and the DC bus capacitor may bepositioned within the AC power adapter 308. For example, the rectifiermay be configured to receive alternating current from the AC powersupply via lines ACH and ACL, and output rectified voltage to the powertool 300 by way of lines DC+ and DC−. Accordingly, the AC power adapterand the power supply interface may only include terminals B+ and B−,which electrically couple the rectifier to the DC bus of the power tool.Thus, the terminal AC for supply alternating current to the power toolis no longer required. It should be readily understood to one skill inthe art that the rectifier and the DC bus capacitor 110 can beimplemented in various suitable ways within the AC power adapter. Forexample, the rectifier, the DC bus capacitor, and the auxiliarycapacitor circuit can be provided on a single circuit or on one or morecircuits. Furthermore, the DC bus capacitor may be provided with thepower tool whereas the rectifier is provided with the AC power adapter.

The AC power adapter 308 covers the DC battery interfaces 304 of thepower supply interface 302 when AC power is being supplied to the powertool 300. In addition, the auxiliary switch path 150 and the switchcontrol circuit 156 are physically positioned external to the power tool300 within the AC power adapter 308. By placing the auxiliary capacitor152, the switch 154, and the switch control circuit 156 within theseparate AC power adapter 308, the circuit for the power tool 300 may bemore compact and is limited to the essential components powering theelectric motor 28.

In particular, when DC batteries are used to power the power tool 300,the power tool 300 does not require the auxiliary capacitor 152. Theauxiliary capacitor 152 is provided only for AC power. Accordingly, inaddition to the performance benefits outlined with respect to the firstembodiment, the auxiliary capacitor circuit 330 that includes theauxiliary switch path 150 and the switch control circuit 156 reduce thecomplexity of the power tool 300 without comprising performance of thepower tool.

As discussed above, in an embodiment, the auxiliary switch path and theswitch control circuit may be positioned outside of the housing of thepower tool and within an adaptor that is configured to connect to thepower tool. In the exemplary embodiment of FIGS. 12-17, the auxiliaryswitch path is coupled via the adaptor across the DC bus line of thepower tool. In an alternative embodiment, as described herein, analternative auxiliary switch path is described that is placed across theAC power line of the adaptor.

FIG. 12 depicts an exemplary power system 400 including power tool 10previously described with reference to FIGS. 1 and 2, an adaptor 410disposed between the power tool 10 and an AC power supply, and an ACpower plug 412 to be inserted into a corresponding plug receptacle ofthe AC power supply.

FIG. 13 depicts a circuit block diagram of the power system 400,according to an embodiment. In this figure, as previously described withreference to FIG. 2, power tool 10 includes a motor 28, position sensor114, and a motor control circuit 100. Adaptor 410 is disposed betweenthe AC power supply 116 and the power tool 10, supplying electric powerto the rectifier 108. Adaptor 410 includes an auxiliary capacitorcircuit, as described herein, disposed on the AC power line. In anembodiment, the auxiliary capacitor circuit may be configured toactivate when, for example, voltage transients on the AC power lineexceed a predetermined voltage threshold. This arrangement places theauxiliary capacitor circuit outside the power tool. Thus, the power toolneed not be modified to include the auxiliary capacitor and theassociated circuitry. Also, this arrangement allows the auxiliarycapacitor circuit to place an auxiliary capacitor across the AC powerline directly. Thus, the power tool need not provide the adaptor 410access to the DC bus.

FIG. 14 depicts an exemplary circuit diagram of an auxiliary capacitorcircuit 500 disposed within the adaptor 410 on the AC power lineidentified herein by Vs and Gnd, according to an embodiment of theinvention. In this embodiment, since the auxiliary capacitor circuit 500is disposed on the AC power line, it is provided with a first auxiliaryswitch path 510 and a first switch control circuit 530 associated withthe positive half cycles of the AC power line, and a second auxiliaryswitch path 520 and a second switch control circuit 540 associated withthe negative half cycles of the AC power line.

In an embodiment, the first auxiliary switch path 510 includes a firstauxiliary capacitor 512 provided in series with a first switch 514(including a transistor 516 and an anti-parallel diode 518) across theAC power line (i.e., between the Vs and Gnd nodes). Similarly, thesecond auxiliary switch path 520 includes a second auxiliary capacitor522 provided in series with a second switch 524 (including a transistor526 and an anti-parallel diode 528) across the AC power line. A firstswitch control circuit 530, which similarly to switch circuit 156 ofFIGS. 7 and 8 includes a first sensing circuit 532 and a comparator 534,is provided to selectively activate or deactivate the first auxiliarycapacitor 512 via the first switch 514. Similarly, a second switchcontrol circuit 540, including a second sensing circuit 542 and a secondcomparator 544, is provided to selectively activate or deactivate thesecond auxiliary capacitor 522 via the second switch 524.

In an embodiment, as shown in FIG. 15, during the positive half cyclesof the AC power line, where current flows through the load (i.e., powertool motor) in a current path indicated by arrow 554, the diode 518 ofthe first switch 514 forms a charge path, indicated by arrow 550 acrossthe first auxiliary capacitor 512. When the first switch control circuit530 detects an over-voltage condition on the AC power line (e.g., whenthe voltage across the first auxiliary capacitor 512 exceeds a voltagethreshold V_(CLAMP+)), it activates the transistor 516 of the firstswitch 514, which it turns provides a discharge path, indicated by arrow552 for the first auxiliary capacitor 512.

Similarly, during the negative half cycles of the AC power line, wherecurrent flows through the load in a current path indicated by arrow 564,the diode 528 of the second switch 524 forms a charge path, indicated byarrow 560 across the second auxiliary capacitor 522. When the secondswitch control circuit 540 detects an over-voltage condition on the ACpower line (e.g., when the voltage across the second auxiliary capacitor522 exceeds a voltage threshold V_(CLAMP−)), it activates the transistor526 of the second switch 524, which it turns provides a discharge path,indicated by arrow 562 for the second auxiliary capacitor 522.

In this manner, the first and second auxiliary capacitors 512 and 514discharge built-up charge to reduce voltage transients presents on theAC power line.

FIG. 16 depicts an exemplary circuit diagram of an alternative auxiliarycapacitor circuit 600 disposed within the adaptor 410 on the AC powerline identified herein by Vs and Gnd, according to an embodiment of theinvention. In this embodiment, the auxiliary capacitor circuit 600 isprovided with one auxiliary capacitor 612 (or multiple capacitorsprovided in parallel) and a corresponding switch control circuit 630. Inan embodiment, switch control circuit 630 includes a sensing circuit 634disposed across the auxiliary capacitor 612 (i.e., on the AC powerline), and a comparator 634, similar to switch control circuit 156 ofFIGS. 7 and 8 previously described. The comparator 634 compares thevoltage across the auxiliary capacitor 612 (i.e., on the AC power line)to a clamp voltage threshold and accordingly outputs a CL signal,described below, to activate an auxiliary switch path allowing theauxiliary capacitor 612 to discharge.

In order to account for the positive and negative half-cycles of the ACpower line, in an embodiment, the auxiliary capacitor circuit 600includes four switches 620, 622, 624, and 626 configured in an H-bridgecircuit arrangement. Each of the switches 620, 622, 624, and 626includes a transistor Q and an anti-parallel diode D, similar to switch150 of FIG. 7 previously described. In an embodiment, switch controlcircuit 630 is configured to control the switching operation of theswitches 620, 622, 624, and 626 to provide an auxiliary switch paththrough switches 620 and 622, or switches 624 and 626.

In order to control the switching operation of the switches in theappropriate half cycle, in an embodiment, switch control circuit 630additionally includes a half-cycle detection and switch control unit 640coupled to the AC power line to detect when the AC power line is in itspositive or negative half cycle. The half-cycle detection unit 640 alsoreceives the CL signal from the comparator 634, and outputs drivesignals G+ and G− to drive the gates of the transistors within theswitches 620, 622, 624, and 626. Based on the CL signal from thecomparator 634, and depending on whether the AC power line is in apositive or a negative half cycle, the half cycle detection and switchcontrol unit 640 turns two of the four switches 620, 622, 624, and 626ON in tandem. Specifically, when CL signal indicates an over-voltagecondition and the AC power line is in a positive half cycle, thehalf-cycle detection and switch control unit 640 activates the G+ signalto turn ON the transistors Q within the switches 620 and 622. Similarly,when CL signal indicates an over-voltage condition and the AC power lineis in a negative half cycle, the half-cycle detection and switch controlunit 640 activates the G− signal to turn ON the transistors within theswitches 624 and 626.

In an embodiment, as shown in FIG. 17, during the positive half cyclesof the AC power line, where current flows through the motor in a currentpath indicated by arrow 654, the diodes of the 620 and 622 forms acharge path, indicated by arrow 650 across the auxiliary capacitor 612.This charge path 650 flows from Vs, through nodes 602 and 604, diode Dof switch 622, capacitor 612, diode D of switch 620, and nodes 606 and608, to Gnd. When the switch control circuit 630 detects an over-voltagecondition on the AC power line (e.g., when the voltage across theauxiliary capacitor 612 exceeds a voltage threshold V_(CLAMP)), itactivates the transistors Q in switches 620 and 622, which it turnsprovides a discharge path, indicated by arrow 652 for the auxiliarycapacitor 512, substantially opposite the charge path 650.

Similarly, in an embodiment, as shown in FIG. 18, during the negativehalf cycles of the AC power line, where current flows through the motorin a current path indicated by arrow 664, the diodes of the 624 and 626forms a charge path, indicated by arrow 660 across the auxiliarycapacitor 612. This charge path 660 flows from Gnd, through nodes 608and 606, diode D of switch 626, capacitor 612, diode D of switch 624,and nodes 604 and 602, to Vs. When the switch control circuit 630detects an over-voltage condition on the AC power line (e.g., when thevoltage across the auxiliary capacitor 612 exceeds a voltage thresholdV_(CLAMP)), it activates the transistors Q in switches 624 and 626,which it turns provides a discharge path, indicated by arrow 662 for theauxiliary capacitor 512, substantially opposite the charge path 660.

FIG. 19 depicts an exemplary block circuit diagram of half-cycledetection and switch control unit 640, according to an embodiment. In anembodiment, half-cycle detection and switch control unit 640 includes apositive half-cycle crest detector unit 644 and a negative half-cyclecrest detector unit 644 respectively coupled to Vs and Gnd signals andconfigured to output an ON signal when positive or negative half cycleis detected. These crest detectors may be designed according to anyknown half cycle crest detection mechanism, as will be understood bythose skilled in the art. Crest detectors remain ON as long as an AChalf cycle is detected and until the next zero crossing. The output ofpositive half-cycle crest detector unit 642 is provided to an AND gate646 along with the CL signal from the comparator 634. Similarly, theoutput of negative half-cycle crest detector unit 644 is provided to anAND gate 648 along with the CL signal. The outputs of AND gates 646 and648 are respectively outputted on G+ and G− nodes.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment but, where applicable, are interchangeable and can be used ina selected embodiment, even if not specifically shown or described. Thesame may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

What is claimed is:
 1. A power tool system configured to receive analternating current from an alternating current (AC) power supply, thepower tool system comprising: an electric motor; a switching circuithaving a plurality of motor switches connected electrically between theAC power supply and the electric motor to regulate supply of power tothe electric motor; a first switch path associated with a positive halfcycle of the alternating current coupled across the AC power supply, thefirst switch path including a first switchable capacitor arranged inseries with a first switch, wherein a state of the first switch controlsa first discharging path for the first switchable capacitor within thepositive half cycle of the alternating current; and a second switch pathassociated with a negative half cycle of the alternating current coupledacross the AC power supply parallel to the first switch path, the secondswitch path including a second switchable capacitor arranged in serieswith a second switch, wherein a state of the second switch controls asecond discharging path for the second switchable capacitor within thenegative half cycle of the alternating current.
 2. The power tool systemof claim 1, further comprising a rectifier arranged between the AC powersupply and the switching circuit to output a rectified signal on a DCpower bus line.
 3. The power tool system of claim 2, further comprisinga DC bus capacitor electrically coupled in parallel with the rectifierand electrically connected between the rectifier and the switchingcircuit, wherein a capacitance of the DC bus capacitor is less acapacitance of the first switchable capacitor and a capacitance of thesecond switchable capacitor.
 4. The power tool system of claim 3,wherein the capacitance of the DC bus capacitor is about 5 μF to 20 μF.5. The power tool system of claim 1, further comprising: a first switchcontrol circuit configured to detect a first voltage associated with thepositive half cycle of the alternating current and control the state ofthe first switch in accordance with a magnitude of the first voltage;and a second switch control circuit configured to detect a secondvoltage associated with the negative half cycle of the alternatingcurrent and control the state of the second switch in accordance with amagnitude of the second voltage.
 6. The power tool system of claim 5,wherein the first switch control circuit includes a first sensingcircuit and a first comparator, the first sensing circuit beingconfigured to detect the first voltage, and the first comparator beingconfigured to compare the first voltage with a first predeterminedvoltage threshold and output a first signal to the first switch tocontrol the state of the first switch accordingly; and wherein thesecond switch control circuit includes a second sensing circuit and asecond comparator, the second sensing circuit being configured to detectthe second voltage, and the second comparator being configured tocompare the second voltage with a second predetermined voltage thresholdand output a second signal to the second switch to control the state ofthe second switch accordingly.
 7. The power tool system of claim 1,wherein the electric motor is a brushless motor.
 8. The power toolsystem of claim 1, wherein the first discharging path is opposite thesecond discharging path.
 9. The power tool system of claim 1,comprising: a power tool including a first housing that houses theelectric motor, the switching circuit, a controller controlling aswitching operation of the switching circuit; and an adapter including asecond housing that houses the first switch path and the second switchpath.
 10. The power tool system of claim 1, wherein the first switchcomprises a first anti-parallel diode forming a charging path for thefirst switchable capacitor and the second switch comprises a secondanti-parallel diode forming a charging path for second switchablecapacitor.
 11. A power adapter configured to supply electric power froman alternating current from an alternating current (AC) power supply toa powered apparatus, the power adapter comprising: a housing; and aswitchable capacitor circuit housed within the housing and including: afirst switch path associated with a positive half cycle of thealternating current coupled across the AC power supply, the first switchpath including a first switchable capacitor arranged in series with afirst switch, wherein a state of the first switch controls a firstdischarging path for the first switchable capacitor within the positivehalf cycle of the alternating current; and a second switch pathassociated with a negative half cycle of the alternating current coupledacross the AC power supply parallel to the first switch path, the secondswitch path including a second switchable capacitor arranged in serieswith a second switch, wherein a state of the second switch controls asecond discharging path for the second switchable capacitor within thenegative half cycle of the alternating current.
 12. The power adapter ofclaim 11, further comprising: a first switch control circuit configuredto detect a first voltage associated with the positive half cycle of thealternating current and control the state of the first switch inaccordance with a magnitude of the first voltage; and a second switchcontrol circuit configured to detect a second voltage associated withthe negative half cycle of the alternating current and control the stateof the second switch in accordance with a magnitude of the secondvoltage.
 13. The power adapter of claim 12, wherein the first switchcontrol circuit includes a first sensing circuit and a first comparator,the first sensing circuit being configured to detect the first voltage,and the first comparator being configured to compare the first voltagewith a first predetermined voltage threshold and output a first signalto the first switch to control the state of the first switchaccordingly; and wherein the second switch control circuit includes asecond sensing circuit and a second comparator, the second sensingcircuit being configured to detect the second voltage, and the secondcomparator being configured to compare the second voltage with a secondpredetermined voltage threshold and output a second signal to the secondswitch to control the state of the second switch accordingly.
 14. Thepower adapter of claim 11, wherein powered apparatus is a power toolhaving a brushless DC motor powered by the electric power.
 15. The poweradapter of claim 11, wherein the first discharging path is opposite thesecond discharging path.
 16. The power adapter of claim 11, wherein thefirst switch comprises a first anti-parallel diode forming a chargingpath for the first switchable capacitor and the second switch comprisesa second anti-parallel diode forming a charging path for secondswitchable capacitor.